1. Field of the Invention
The present invention relates to a noise reduction circuit available in a video signal processing apparatus and in particular to a noise reduction circuit for video contour emphasis signals which can eliminate properly the noise signal mixed into the video contour signal and amplified together when the video contour signal for any video system will be emphasized.
2. Description of the Prior Art
Generally, a noise reduction apparatus is usually used in a signal processing circuit for the purpose of improving the signal to noise ratio.
In the video system, when the contour signal is emphasized, the noise signal is amplified synchronously with the contour signal, and thereby the signal to noise ratio is deteriorated.
Therefore, it is necessary to provide a noise reduction means for the video contour emphasis signal.
As for an example in the conventional noise reduction means for improving the S/N ratio, there has been proposed a limiter circuit comprising a pair of diodes D1 and D2 connected in parallel and in opposite polarity to each other as shown in FIG. 1A. The limiter circuit has the noise reduction range `a` inclusive of the cut-in voltages or the threshold voltages VD1 and VD2 (FIG. 1B) of the respective diodes D1 and D2 and shows the input/output characteristics shown in FIG. 1C so that the noise signal having an amplitude in the range `a` is allowed to be zero.
The above-mentioned prior art circuit, however, is adapted to limit the noise reduction level within the cut-in voltage VD1 and VD2 as shown in FIG. 1B and 1C, because the cut-in voltages (or threshold voltage) of the respective diodes are fixed and not variable.
Thus it is not possible to change the noise reduction level according to the change of amplitudes of the video contour signal and the noise signal, and so the noise signal having the amplitude beyond the range `a` shown in FIG. 1C is not effectively reduced.
Further, U.S. Pat. No. 4,571,548 discloses a limiter circuit comprising buffer amplifiers, a low pass filter, and a pair of diodes connected in parallel and in opposite polarity between the input terminal of the low pass filter and the output terminal of the buffer amplifier, wherein the voltage limit is established by the forward voltage drop of two parallel, oppositely-connected diodes which form part of the limiter circuit so as to eliminate the interference signal from the desired signal.
The above limiter circuit, however has shortcomings in that voltage limit against the interference signal also falls into the range based on the forward voltage drop of the respective diodes.